The present invention generally relates to a method for controlling emissions, and more particularly relates to a system and method for controlling emissions in a parallel hybrid electric motor vehicle by controlling manifold absolute pressure.
Hybrid electric vehicles achieve high fuel mileage by combining a battery-powered electric motor/generator (MG) with a highly efficient heat engine, typically an internal combustion engine (ICE). Parallel hybrid electric vehicles use power from both the heat engine and the electric engine to drive the wheels of the vehicle. This is in contrast to series hybrid electric vehicles, which only use power from the electric motor to drive the wheels, with the heat engine only acting as a generator to recharge the electric motor/generator batteries. Parallel hybrid electric vehicles are the most common hybrid electric motor vehicles, and are the focus of the present invention. By using on-board engine computer controls to vary when each engine or motor or combination of engine and motor are used, the parallel hybrid motor vehicle can achieve peak efficiency under different driving conditions. The motor/generator functions as both a motor, delivering torque through some mechanism to the drive wheels, and as a generator, powering the electrical system of the vehicle and charging the vehicle batteries. When the MG is functioning as a generator, it may either be powered by torque from the motor vehicle ICE or from the wheels of the motor vehicle.
When the internal combustion engine (ICE) in any motor vehicle is initially started (especially in a cold climate), the interior surfaces of the engine are cold. In addition, because the engine is initially turning at a very low RPM, the intake manifold absolute pressure (MAP) is near atmospheric pressure. Because liquid fuel does not combust as easily or cleanly as gaseous fuel, it is desirable that the fuel sprayed into and mixed with air traveling into the combustion cylinders of the engine be vaporized in order to reduce emissions from the ICE. Unfortunately, both the relatively high MAP and the cold condition of the engine make it difficult to vaporize the fuel injected into the combustion cylinders. Therefore, in order to produce the desired amount of power at start up and during high-torque initial accelerations shortly after start up when the engine is still cold, additional (i.e., excess) amounts of fuel must be supplied to the intake manifold to obtain a sufficient amount of vaporized fuel. All of the additional fuel is not completely vaporized and the incompletely vaporized fuel is not completely combusted. The consequence of the poor fuel vaporization at startup and during initial high-torque accelerations is increased emissions. The excess fuel that is not completely combusted at start-up and during the period shortly after start up creates an exhaust mixture that is too fuel-rich to be stoichiometric at the catalytic converter, thus leading to increased hydrocarbon and carbon monoxide emissions. If an attempt is made to decrease the emissions by not supplying the additional amounts of fuel, the engine may misfire because there is not enough fuel vapor to run correctly. If the engine misfires, in addition to providing substandard performance, not all the fuel injected into the combustion chambers burns, and the exhaust passed to the catalytic converter is again too rich to be stoichiometric. This situation also leads to increased hydrocarbon emissions.
Under most operating conditions, once the intake valves have become adequately heated, the excess fuel is no longer necessary, as the heated intake valves will properly vaporize the injected fuel. At the same time the engine RPM is also usually high enough to provide a low MAP which assists with fuel vaporization.
High emissions can result, however, not only from the high MAP at start up, but also from rapid changes in MAP even with the engine heated. When there is a rapid drop in torque demand, such as at the end of a rapid acceleration, the throttle closes and the MAP will quickly drop from the high MAP consistent with the rapid acceleration to a low MAP consistent with the lower torque demand. Any liquid fuel left in the intake ports after the throttle closes rapidly can flash to a gaseous state because of the low MAP and the hot engine components. There may be too little of this gaseous fuel to fully combust, and the fuel-air mixture may be too lean (has too much air present) to properly and completely ignite in the cylinder combustion chamber. The unburned fuel-air mixture is exhausted and passes to the catalytic converter. This unburned fuel-air mixture again leads to increases in the hydrocarbon and carbon monoxide emissions. This problem can be especially pronounced when the engine is cold, such as in the short period after startup. During this period, some of the excess fuel that has been injected into the combustion chambers can pool on the cylinder intake ports. If the MAP suddenly drops due to the throttle closing, the liquid fuel can quickly flash to a gaseous state, leading to the problems described above.
Air injection reaction (AIR) systems have been employed as one means to reduce the emissions resulting from start-up and from the driving immediately thereafter by pumping air into the exhaust ports. The injected air helps produce an exothermic reaction to increase the catalytic converter temperature. Additionally, advanced engine controls have been used to provide a more easily ignited air/fuel mixture for injection into the cylinders. An approach to controlling emissions on a motor vehicle without a hybrid power train involves the use of an electronic throttle control (ETC). An ETC has a sensor at the accelerator pedal that measures the position of the accelerator pedal, and thus how much power (or torque) is being demanded from the engine by the driver. The sensor is connected to an engine control computer which, based on signals from the accelerator pedal sensor, decides how far to open the throttle flap in the intake manifold, thereby determining the amount of power the engine produces. Under normal operating conditions, the engine control computer immediately opens or closes the throttle in order to obtain the desired engine. The engine control computer may, however, be programmed to slow the opening or closing of the throttle in order to avoid a high-emissions situation that can result from opening or closing the throttle too quickly, as described above. Problems exist, however, with all of these approaches. The AIR system is only used for less than a minute at start-up of the motor vehicle and has no function thereafter in the operation of the motor vehicle. The AIR system adds weight and complexity (and thus cost) to the motor vehicle and yet is only functionally necessary for a short period of time at cold start up. The advanced engine controls also add complexity and cost to the motor vehicle. The first two methods help primarily with reducing start-up and initial emissions and are largely unable to reduce emissions in other driving situations such as conditions resulting in rapid changes in MAP. The method involving the use of an ETC can help reduce emissions based on rapid changes in MAP but may reduce responsiveness of the engine to driver inputs, as torque output from the engine is slowly increased or decreased in order to avoid high emissions.
Accordingly, a need exists for a method to control emissions in a parallel hybrid electric motor vehicle that overcomes the deficiencies of the prior art techniques without sacrificing vehicle performance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
A system is provided for controlling manifold absolute pressure in an internal combustion engine of a hybrid electric vehicle. The system includes an internal combustion engine, an electric motor/generator, and a control unit. The control unit is coupled to the internal combustion engine and to the electric motor/generator and is configured to control the torque supplied by the internal combustion engine and by the electric motor/generator to meet the vehicle total torque demand and to maintain the manifold absolute pressure and the rate of change of the manifold absolute pressure measured in the intake manifold of the internal combustion engine within acceptable emission control limits.
In accordance with a further embodiment of the invention, a method is provided for controlling manifold absolute pressure in a hybrid electric vehicle that includes an internal combustion engine in parallel with an electric motor/generator. The method includes the steps of monitoring the torque demand on the hybrid electric vehicle, monitoring the manifold absolute pressure of the internal combustion engine, supplying torque from the internal combustion engine to meet the torque demand; and supplying torque from the motor/generator to load-level the torque supplied from the internal combustion engine and to maintain the manifold absolute pressure of the internal combustion engine within an acceptable range.